Control circuits



April 1963 M. D. BOWERS 3,087,109

CONTROL CIRCUITS Filed Nov. 10, 1959 IN V EN TOR. M cZz/zi Z 5: D.Boa/ens 2 /144! flttorngr 3,687,109 CONTROL CIRCUITS Melville DoreyBowers, Lake Valhalla, Montville, N.J., assignor to McGraw-EdisonCompany, Milwaukee, Win, a corporation of Delaware Filed Nov. 10, 1959,Ser. No. 852,016 6 Claims. (Cl. 323-6) This invention relates to controlcircuits and, more particularly, to a control circuit utilizing a Zenerdiode as a reference of voltage source.

Zener diodes are commonly utilized in control circuits as a referencevoltage source for comparison with a variable input signal so that anoutput signal or eflect can be obtained which is a function of inputsignal variations. Zener diodes have, however, a positive temperaturecoeflicient so that changes in ambient temperature modify the outputsignal or etfect whereby it no longer reflects true variations in theinput signal.

It is an object of this invention to provide a Zener diode with meansfor compensating variations in its Zener voltage with changes in ambienttemperature.

It is a more specific object of the invention to utilize in a controlcircuit, a positive temperature coefficient resistor for temperaturecompensationg a Zener diode.

it is a still more specific object of the invention to utilize in acontrol circuit having a Zener diode, a positive temperature coeflicientresistor and a second resistor having a substantially zero temperaturecoeflicient to compensate for changes in Zener voltage with temperature.

These and other objects and advantages of the invention will become moreobvious from the detailed description of the invention taken in view ofthe accompanying drawing which shows a control circuit embodying theinstant invention.

Referring to the drawing in greater detail, a voltage sensitive bridge10 is shown having a Zener diode 12 as a reference voltage source. Theother legs of the bridge include a pair of fixed resistors 14 and 15 andan adjustable resistor 16. The input terminals 17 and 18 of bridge 10are respectively connected to junction 19 between resistors 14 and 15and junction 20 between the negative terminal of Zener diode 12 andresistor 16 respectively. A positive temperature coeflicient resistor 22is connected at one end to junction 23 between resistor 15 and thepositive terminal of Zener diode 12 while a second resistor 24, having asubstantially zero temperature coeflicient within the normal temperaturerange of the device, is connected between the other end of resistor 22and junction 20. A load 28 is connected to output terminals 26 and 27between resistors 14 and 16 and resistors 22 and 24 respectively,

In operation, resistor 16 is so adjusted that When a predetermineddesired voltage appears across input terminals 17 and 18, the circuit isbalanced whereby the current flowing to load 28 is Zero, and thecurrents flowing in resistors 22 and 2 4 are equal. Under suchconditions the voltage drop across resistor 14 is equal to the sum ofthe voltage drops across resistors 15 and 22. In addition, the voltagedrops across resistors 16 and 24 are also equal. It can be seen too thatthe sum of the voltage drops across resistors 22 and 24 is equal to theZener voltage of diode 12.

Should the input voltage rise above the predetermined desired level, thecurrent through resistors 1-4 and 16 will increase resulting in anincrease in voltage at terminal 26 with respect to terminal 20.Increased current will also flow through resistor 15 and Zener diode 12,which, assuming perfect operation thereof will cause no change in thevoltage between terminals 20 and 23. Thus the current through resistors22 and 24 and the voltage distribution across them would not change ifit were not for the load 28 connecting points 26 and 27. Since thevoltage at terminal 26 has increased, a current will flow in load 28from that terminal to terminal 27 through resistors 24 and 16. This loadcurrent flow will tend to increase the voltage drop across resistor 24and decrease the drop across resistors 16 and 22. The load current thustends to reduce the voltage difference between terminals 26 and 27 whichwould appear if the load resistance were infinite.

On the other hand, should the voltage between input terminals 17 and 18fall below the predetermined desired value, a reduced current would flowthrough resistors 14 and 16 resulting in a decrease in voltage atterminal 26. At the same time, current flow through Zener diode 12 andresistor 15 is reduced but the voltage drop across diode 12 is unchangedtending to maintain the voltage drop across resistors 22 and 24. Now,the voltage drop across resistor 24 is higher than the voltage dropacross resistor 16 and current fiow through load 28 is reversed.

It can therefore be seen that changes in input voltage will be reflectedas a change in the current flowing through load 28. In order that thecurrent flowing through load 28 reflect changes in input voltage only,it is necessary to maintain the voltage at terminal 27 with respect toterminal 20 invariant.

Since Zener diodes have a positive temperature coeflicient, the voltagedrop between terminals 23 and 20 varies with changes in ambienttemperature, even though the input voltage between terminals .17 and 18remains constant. In order to prevent such temperature-produced changesin Zener voltage from being reflected as error signals in load 28, it isnecessary to insure that the current in resistor 24 remains unaifected.This is accomplished by utilizing a resistor 22 having a positivetemperature coeflicient. By properly matching the temperaturecoeflicients of Zener diode 12 and resistor 22, any increase or decreasein the voltage across Zener diode 12 can be matched by an increase ordecrease in the voltage drop across resistor 22. As a result, thevoltage drop across and the current through resistor 24 will remainconstant.

For example, assume that the bridge circuit 10 is balanced and that atemperature rise increases the Zener voltage of diode 12 and hence, thevoltage drop between junctions 20 and 23. Since the current in resistor24, as well as its resistance, must remain constant to satisfy theconditions of compensation, the entire increase in the voltage dropbetween terminals 20 and 23 appears across resistor 22. However, becausethe current in resistor 22 is equal to the current in resistor 24, underbalanced conditions, it therefore also remains constant. Accordingly theincreased voltage drop across resistor 22 results entirely from anincrease in its resistance. It can be seen, therefore, that resistor 22must have a positive temperature coefficient which bears a particularrelation to the temperature coeificient of Zener diode 12.

Those skilled in the art will appreciate that if the resistance of load28 is very large with respect to resistor 24, the current through thelatter will be equal to the Zener voltage of diode 12 divided by the sumof the resistances of resistors 22 and 24, and the voltage acrossresistor 24 will be equal to this current times its resistance R.Because the Zener diode and resistor 22 are temperature sensitive, theexpression for the voltage drop across resistor 24 at a temperature Tis:

R =the resistance of resistor 24; R =the resistance of resistor 22 at Tv.0 V =the Zener voltage of diode 12 at T oc tlle temperaturecoeflicient of Zener diode 12; and or=the temperature coeificient ofresistor 22.

Since there is no change in the resistance or current through resistor24, the voltage drop across it will not change with temperature so thatthe above expression can be differentiated with respect to temperatureand set equal to zero, from which the following expression can beobtained:

This expression indicates that the necessary temperature coeffieient ofresistor 22 is a positive value and can be accurately determined fromthe resistances of resistors 22 and 24 and the temperature coefiicientof Zener diode 2'12.

While the invention is discussed with respect to a voltage sensitivebridge, it will be understood that it has application in any circuit inwhich a Zener diode is utilized as a voltage reference source.

I claim:

1. A circuit for deriving a reference voltage independent of temperaturevariations and including a pair of input terminals connected to avoltage source, a Zener diode and a first resistance means seriesconnected across said input terminals, a second resistance meansconnected at one end to one terminal of said Zener diode, a thirdresistance means connected between the other end of said secondresistance means and the other terminal of said Zener diode, said secondresistance means having a positive temperature coefficient, said thirdresistanec means having a zero temperature co-efficient, whereby areference voltage will be derived across said third resistance meansthat is independent of temperature variations.

2. A circuit for deriving a reference voltage independent of temperaturevariations and including a pair of input terminals connected to avoltage source, a Zener diode and a first resistance means seriesconnected across said input terminals, a second resistance meansconnected at one end to the positive terminal of said Zener diode, athird resistance means connected between the other end of said secondresistance means and the negative terminal of said Zener diode, saidsecond resistance means having a positive temperature coetficient, saidthird resistance means having a zero temperature coefficient, whereby areference voltage will be derived across said third resistance meansthat is independent of temperature variations.

3. A circuit for deriving a reference voltage independent of temperaturevariations and including a pair of input terminals connected to avoltage source, a Zener diode and a first resistance means seriesconnected across said input terminals, a second resistance meansconnected at one end to one terminal of said Zener diode, a thirdresistance means connected between the other end of said secondresistance means and the other terminal of said Zener diode, said thirdresistance means having a zero temperature coefficient, said secondresistance means having a positive temperature coefficient given by theexpression where R =the resistance of said second resistance means at areference temperature;

R =the resistance of said third resistance means; and a =the temperaturecoefficient of said Zener diode.

whereby a reference voltage will be derived across said third resistancemeans that is independent of temperature variations.

4. A bridge type control circuit having a pair of inputterminals and apair of output terminals, a first resistance means connected between oneof said input terminals and one of said output terminals, a secondresistance means connected between the one of said output terminals andthe other of said input terminals, a third resistance means connectedbetween the other of said input terminals and the other of said outputterminals, and a fourth and a fifth resistance means series connected toeach other, said fourth resistance means also being connected to the oneof said input terminals and said fifth resistance means also beingconnected to the other of said output terminals, said third resistancemeans having a substantially zero temperature coefiicient, said fifthresistance means having a positive temperature coefficient, and a Zenerdiode connected between said other input terminal and the junctionbetween said fourth and fifth resistance means.

5. A circuit for deriving a reference voltage independent of temperaturevariations and including a pair of input terminals connected to avoltage source, a diode and a first resistance means series connectedacross said input terminals, a second resistance means connected at oneend to one terminal of said diode, a third resistance means connectedbetween the other end of said second resistance means and the otherterminal of said diode, said diode being characterized by asubstantially constant breakdown potential and a positive temperaturecoeffiient, said second resistance means having a positive temperaturecoefficient, said third resistance means having a zero temperaturecoefiicient, whereby a reference voltage will be derived across saidthird resistance means that is independent of temperature variations.

6. A bridge type control circuit having a first leg con sisting of afirst resistance means, a second leg consisting of a second resistancemeans and connected to said first leg, a third leg consisting of a thirdresistance means and connected to said second leg, and a fourth legconsisting of a fourth and a fifth resistance means series connected toeach other, said fourth resistance means also being connected to saidfirst resistance means and said fifth resistance means also beingconnected to said third resistance means, said third resistance meanshaving a substantially zero temperature coefficient, said fifthresistance means having a positive temperature coeflicient, and a Zenerdiode connected between the junction of said second and third legs andthe junction between said fourth and fifth resistance means.

References Cited in the file of this patent UNITED STATES PATENTSCrowell Aug. 18, 1942 Woodworth Dec. 9, 1958 OTHER REFERENCES

1. A CIRCUIT FOR DERIVING A REFERENCE VOLTAGE INDEPENDENT OF TEMPERATUREVARIATIONS AND INCLUDING A PAIR OF INPUT TERMINALS CONNECTED TO AVOLTAGE SOURCE, A ZENER DIODE AND A FIRST RESISTANCE MEANS SERIESCONNECTED ACROSS SAID INPUT TERMINALS, A SECOND RESISTANCE MEANSCONNECTED AT ONE END TO ONE TERMINAL OF SAID ZENER DIODE, A THIRDRESISTANCE MEANS CONNECTED BETWEEN THE OTHER END OF SAID SECONDRESISTANCE MEANS AND THE OTHER TERMINAL OF SAID ZENER DIODE, SAID SECONDRESISTANCE MEANS HAVING A POSITIVE TEMPERATURE COEFFICIENT, SAID THIRDRESISTANCE MEANS HAVING A ZERO TEMPERATURE CO-EFFICIENT, WHEREBY AREFERENCE VOLTAGE WILL BE DERIVED ACROSS SAID THIRD RESISTANCE MEANSTHAT IS INDEPENDENT OF TEMPERATURE VARIATIONS.